He claims there was a massive explosion that shook the walls, caused part of the ceiling to cave in, and other significant effects. The obvious source is the plane impact. If there were multiple explosions of the magnitude he claims, surely other people would have noted them.
And of course, the standard conspiracy theory caveats apply: Do you really believe that their was a massive conspiracy perpetrated by experts who can coordinate such things at scale, yet are also simultaneously so incompetent that they forgot to wait until the plane impact to start demolishing the building?
It's nonsense. I highlighted the speaking tour to explain how he has perverse incentives to be continue spreading the conspiracy theory.
EDIT: I removed a comment about common sense because it seems to be distracting people from the actual comment